We have demonstrated that an anti-H-2 associated serum eliminates helper T-cells that are required both for proliferative responses to foreign H-2 antigens and differentiation of precursor T-cells into specific effector, killer T-cells. Cell-free supernatants contain a helper factor(s) that can replace the eliminated helper T-cells. The supernatants are derived from MLR-activated cells. Addition of helper factor to anti-serum plus RC treated cultures results in a restoration of immune competence to generate killer T-cells at a level similar to that obtained in cultures treated with normal serum plus RC. Both helper factor and foreign stimulating cells are required for precursor T-cell differentiation into killer T-cells. The helper factor is protein in nature and it collaborates across H-2 barriers. The effects of helper factor on the kinetics of killer T-cell differentiation have also been examined.